The objective of this project is to develop a bench top laser based stable isotope analyzer for 13C for the purpose of studying metabolic pathways. The project will determine the feasibility of developing an analyzer for 13C:12C ratios with at least part per thousand sensitivity using a tunable laser. One important health related application will be the development of a simple diagnostic test for the bacterium Heliobacter pylori, which infects virtually all people with duodenal ulcers and about 80% of those with stomach ulcers, replacing the radioactive 14C or mass spectrometer detected 13C labeled urea breath tests. The assay will be based on isotope shifts in the energy levels of the CO2 molecule. A low pressure (~1 torr) cell with exhaled breath will be probed by selected isotopic resonance lines using a low power (~100 mW) tunable laser. Laser transitions and operating powers will be chosen to enhance the 13C:12C sensitivity with 10- 4 ratios reasonably anticipated. Test cells working with isotopically labeled CO2,N2,He mixtures will first be studied to optimize sensitivity and for calibration. Success with the prototype system will then lead to expanded research on other stable isotope assays and implementation of easy to operate instrumentation for research and clinical use.